With your permission, Mr Speaker, I shall make a statement about the government's decision to maintain the United Kingdom's independent nuclear deterrent.

There are many complex technical, financial and military issues to be debated in respect of this decision. But none of them obscure or alter the fundamental political judgment at the crux of it.

Britain has had an independent nuclear deterrent for the last half century. In that time the world has changed dramatically, not least in the collapse of the Soviet Union, the original context in which the deterrent was acquired.

Given that this change has occurred, the question is whether it is wise to maintain the deterrent in the very different times of today.

The whole point about the deterrent is not to create the circumstances in which it can be used but on the contrary to try to create circumstances in which it is never used.

Necessarily, therefore, any analysis of what role it could play in a situation that is hypothetical will always be open to the most strenuous dispute.

Ultimately, this decision is a judgment, a judgment about possible risks to our country and its security, and the place of the deterrent in thwarting those risks.

The government's judgment, on balance, is that, though the cold war is over, we cannot be certain in the decades ahead that a major nuclear threat to our strategic interests will not emerge, that there is also a new and potentially hazardous threat from states such as North Korea which claims already to have developed nuclear weapons or Iran which is in breach of its non-proliferation duties, that there is a possible connection between some of those states and international terrorism, that it is noteworthy that no present nuclear power is or is even considering divesting itself of its nuclear capability unilaterally, and that in these circumstances, it would be unwise and dangerous for Britain, alone of any of the nuclear powers, to give up its independent nuclear deterrent.

Notice that I do not say that the opposite decision is unthinkable, or that anyone who proposes it is pacifist or indifferent to our country's defence.

There are perfectly respectable arguments against the judgment we have made.

I both understand them and appreciate their force.

It is just that, in the final analysis, the risk of giving up something that has been one of the mainstays of our security since the war, and moreover doing so when the one certain thing about our world today is its uncertainty, is not a risk I feel we can responsibly take.

Our independent nuclear deterrent is the ultimate insurance.

It may be, indeed hopefully is the case that the eventuality against which we are insuring ourselves, will never come to pass.

But in this era of unpredictable but rapid change, when every decade has a magnitude of difference with the last, and when the consequences of a misjudgement on this issue would be potentially catastrophic, would we want to drop this insurance and not as part of a global move to do so, but on our own? I think not.

However, what will happen from today will be a very full process of debate.

It is our intention, at the conclusion of that process in March of next year, to have a vote in this house.

We will make arrangements during the process to answer as fully as possible any of the questions that arise.

And of course, I am sure the Defence Select Committee, at least, will want to examine the issue carefully.

The white paper, which we publish today, goes into, not merely the reasons for the decision, but also a technical explanation of the various options and tries to cover in some detail all potential lines of dispute or inquiry.

I hope therefore that we can focus on the decision itself not the process.

Let me now turn to some of the key questions.

First, the reason this decision comes to us now, is that if in 2007 we do not take the initial steps toward maintaining our deterrent, shortage of time may prevent us from being able to do so.

Necessarily we can only form this view based on estimates, but these are from the evidence given to us by our own experts, by the industry that would build the new submarines and the experience of other nuclear states.

Our deterrent is based on four submarines.

At any one time, one will be in dock undergoing extensive repair and maintenance, usually for around four years.

The other three will be at sea or in port for short periods.

But at all times at least one will be on deterrent patrol, fully armed.

The submarines are equipped with Trident D5 missiles that are US-manufactured but maintained with our close technical and scientific collaboration.

The operation of the system is fully independent - a missile can be fired only on the instructions of the British prime minister.

The current Vanguard submarines have a service life of 25 years.

The first boat should leave service in 2017.

We can extend that for five years.

In 2022, that extension will be concluded and in 2024 the second boat will also end its extended service life.

By this time, we will only have two Vanguard submarines.

This will be insufficient to guarantee continuous patrolling.

The best evidence we have is that it will take us 17 years to design, build and deploy a new submarine.

Working back from 2024, that means we have to take this decision in 2007.

Of course, all these timelines are estimates, but they conform to the experience of other countries with submarine deterrents as well as our own.

Secondly, we have looked carefully at the scope of different options.

The white paper sets them out.

Aircraft with cruise missiles... but cruise missiles travel at subsonic speeds and building the special aircraft would be hugely expensive.

A surface ship equipped with Trident... but a far easier target.

A land-based system with Trident... but in a small country like the United Kingdom immensely problematic and also again an easier target.

There is no real doubt on this score: if you want an independent nuclear deterrent, for a nation like the UK, a submarine-based one is best.

It is also our only deterrent.

In the 1990s we moved to Trident as our sole nuclear capability.

Of the other major nuclear powers, the US has submarine, air and land-based capability.

Russia has all three capabilities and has the largest number of nuclear weapons.

France has both submarine and air-launched capability and has a new class of submarines in development the last of which is due to come into service in 2010.

China has a smaller number of land-based strategic nuclear weapons but is working on modernising its capability including a submarine-based nuclear ballistic missile.

We will continue to procure some elements of the system, particularly those relating to the missile, from the US.

But, as now, we will maintain full operational independence.

The submarines, missiles, warheads, and command chain are entirely under British control, and will remain so after 2024.

This gives British prime ministers the necessary assurance that no aggressor can escalate a crisis beyond UK control.

A new generation of submarines will make maximum use of existing infrastructure and technology.

The overall design and manufacture costs - of 15-20 billions - are spread over three decades, are on average 3% of the defence budget, and are at their highest in the early 2020s.

As before, we will ensure that the investment required will not be at the expense of the conventional capabilities our armed forces need.

It is our intention that the procurement and building will, as now, be done by British industry, with thousands of British, highly-skilled jobs involved.

However, we will investigate whether, with a new design, we can maintain continuous patrol with a fleet of only three submarines.

A decision on this will be made once we know more about the submarines' detailed design.

No decisions are needed now on the warhead.

We can extend the life of the D5 Trident missile to 2042.

After that, there will be the opportunity for us to participate in any new missile design in collaboration with the US, something which will be confirmed in an exchange of letters between myself and the President of the USA.

Maintaining our nuclear deterrent capability is also fully consistent with all our international obligations.

We have the smallest stockpile of nuclear warheads amongst the recognised nuclear weapons states, and are the only one to have reduced to a single deterrent system.

Furthermore, we have decided, on expert advice, that we can reduce our stockpile of operationally available warheads to no more than 160, which represents a further 20 per cent reduction.

Compared with previous plans, we will have reduced the number of such weapons by nearly half.

It is written as a fact by many that there is no possibility of nuclear confrontation with any major nuclear power.

Except that it isn't a fact.

Like everything else germane to this judgment, it is a prediction.

It is probably right. But certain? No, we can't say that.

The new dimension is undoubtedly the desire by states, highly dubious in their intentions, like North Korea and Iran, to pursue nuclear weapons capability.

Fortunately, Libya has given up its WMD ambitions and has played a positive role internationally; the notorious network of A Q Khan, the former Pakistani nuclear physicist has been shut down.

But proliferation remains a real problem.

The notion of unstable, usually deeply repressive and anti-democratic states, in some cases profoundly inimical to our way of life, having a nuclear capability, is a distinct and novel reason for Britain not to give up its capacity to deter.

It is not utterly fanciful either to imagine states sponsoring nuclear terrorism from their soil.

It is not impossible to contemplate a rogue government helping such an acquisition.

It is true that our deterrent would not deter or prevent terrorists.

But it is bound to have an impact on governments that might sponsor them.

Then there is the argument, attractive to all of us who believe in the power of countries to lead by example, as we seek to do in climate change and did in debt relief, that Britain giving up its deterrent, would encourage others in the same direction.

Unfortunately there is no evidence that any major nuclear power would follow such an example; on the contrary.

And, as for the new, would-be nuclear powers, it really would be naive to think that they would be influenced by a purely British decision.

More likely, they would construe it as weakness.

Finally, there is one other argument: that we shelter under the nuclear deterrent of America.

Our cooperation with America is rightly very close.

But close as it is, the independent nature of the British deterrent is again an additional insurance against circumstances where we are threatened but America is not.

These circumstances are also highly unlikely but I am unwilling to say they are non-existent.

In the end, therefore, we come back to the same judgment.

Anyone can say that the prospect of Britain facing a threat in which our nuclear deterrent is relevant is highly improbable.

No one can say it is impossible. In the early 21st century, the world may have changed beyond recognition, since the decision taken by the Atlee government over half a century ago.

But it is precisely because we could not have recognised then, the world we live in now, that it would not be wise to predict the unpredictable in the times to come.

That is the judgment we have come to.

We have done so according to what we think is in the long-term strategic interests of our nation and its security and I commend it to the house.